The present invention relates to improvements in apparatus for controlling the position of a payload along a linear trajectory. It is particularly useful in the field of data recording and playback for controlling the radial position of a write/read head relative to the recording surface(s) of data storage disks (e.g. magnetic, magneto optic, and optical disks) so that information can be written on (or read from) a desired data track.
Heretofore, a variety of radial access mechanisms have been proposed and utilized to control the radial position of a write/read head in a disk recorder/player. Typically, such mechanisms function to advance the write/read head along arcuate or linear trajectories in a plane parallel to the plane of disk rotation. One such mechanism is disclosed in U.S. Pat. No. 4,763,314 issued to M. J. McCaslin et al. on Aug. 9, 1988. Here, the write/read head is moved along a linear trajectory by an electro magnetic actuator comprising a pair of spaced, parallel rods which extend generally transverse to the data tracks on a data storage disk. Each of the rods is made of a ferromagnetic material and, together, the rods function to guide a write/read head bearing carriage along a straight trajectory. A pair of actuator coils are connected to the carriage, each coil surrounding one of the rods for linear movement therealong. Each of the ferromagnetic rods is partially surrounded by a generally C-shaped, radially polarized permanent magnet, each magnet being spaced from the rod so as not to obstruct movement of the carriage along the rod. Upon applying a suitable current to the coils, the coils and their associated carriage move through the magnetic field established by the magnets and, as already noted, are guided along the straight trajectory defined by the ferromagnetic rods.
The linear actuator disclosed in the above-referenced patent is relatively compact in size owing to the fact that the ferromagnetic rods serve to both guide the carriage along the straight trajectory, and to provide a return path for the magnetic flux produced by the permanent magnets. (Note, in many other linear actuators proposed heretofore, these two functions are provided by separate elements.) Though compact in size, linear actuators of this type suffer certain disadvantages. For example, because of the construction and location of the permanent magnets, only about half of the area of the actuator coils is subjected to the magnetic field. This has the effect of decreasing the efficiency of the motor. Also, since two rods are used to guide carriage, their location in relation to each other and to the carriage which they support becomes very critical and requires a very precise assembly procedure. Moreover, because there are two ferromagnetic bars and associated coils, there are, in essence two electromagnetic (voice coil-type) motors which must be balanced in operation.